Unwanted emissions may be considered as electromagnetic disturbances which can cause interference with other signals from e.g. wireless communication services. Nowadays, the frequency allocation of most wireless communications is strictly assigned and shared, and high frequency wireless systems are becoming more popular. The manufacturer of equipment which radiates electromagnetic signals is often mainly responsible for any caused interference. Therefore, it is important to define the level of out-of-band and spurious emissions generated by a radio equipment. For example, the emissions may be coming from an antenna and radio equipment in a base station of a wireless communications network. The measurement of the level of out-of-band and spurious emissions may be used for type approval or on production lines in order to satisfy an electromagnetic interference (EMI) compliance tests and regulatory requirements. Meeting such requirements minimizes interference on other equipment.
FIG. 1 shows an example emissions spectrum 100, which comprises both wanted emissions 102 (in-band) and unwanted emissions 110. The in-band frequency range is used for communication of data, and may be referred to as an operation bandwidth. The unwanted emissions 110 may be considered as comprising spurious emissions 112 and out-of-band emissions 114. The in-band emissions are within an operation bandwidth. The operation bandwidth may be used for radio access communication, e.g. between a base station and a wireless terminal. In this example, the unwanted emissions 110 comprising spurious emissions 112 and out-of-band emissions 114 occur at frequencies above and below the in-band emissions 102.
Spurious emissions 112 may be defined as emissions which are outside the operation bandwidth. In some examples, their level might not directly affect the transmission of information in the operations bandwidth. The spurious emissions may comprise contributions from:                Harmonic emissions: Emissions at frequencies which are multiples of the center frequency emissions        Parasitic emissions: Emissions accidentally generated at frequencies which are independent of the carrier or frequency of an emission.        Intermodulation products: Spurious intermodulation products resulting from the oscillations at the frequency of an emission.        
Out-of-band emissions 114 are in a range adjacent in frequency, e.g. immediately outside, the bandwidth of operation.
Conventionally, in-band emissions are measured at an Antenna Reference Point (ARP) using a cable connected to measurements instruments, a so-called conductive measurement. Measurements outside in-band are performed with a passive load connected to the ARP, i.e. all in-band energy is absorbed in the passive load. A desire for higher data rates and for higher capacity has led to the advance of high integrated antenna systems, for example Active Antenna System (AAS), which may have beam-forming capability. A further example may be a Massive MIMO (Multiple-Input-Multiple-Output) antenna, in which the antenna is integrated within the radio unit in one single module without connectors.
Such antenna types may not provide physical access to an antenna output port, or the number of ARP connectors is too large (e.g. in very large array antennas with hundreds of elements). This may make access difficult to the antenna to perform conductive measurements.
In order to measure the system performance, an Over The Air (OTA) test configuration may be used. An OTA test may also be used for measuring the unwanted emissions radiated by the whole system when the transmitter is on.
Measurements of radiated unwanted emissions are done in an indoor controllable environment, for example in an anechoic chamber or reverberation chamber. An issue of an OTA test results from the high radio frequency (RF) output power (in-band signal) radiated inside the chamber by the Device Under Test (DUT). The power of the in-band signal may cause Passive Intermodulation (PIM) within the test range, thus creating non-representative signals. The in-band signal may interfere with the radiated unwanted emissions in the out-of-band/spurious region, leading to unreliable results. In some cases, the in-band signal may block, or even destroy, the sensitive measuring equipment in the test chamber.
An improved measurement method and system is desired for measurement of unwanted emissions in an OTA test.